1. Field of the Invention
The invention relates generally to welding arrangements and more particularly to a circuit arrangement for controlling welding operations.
2. Description of Related Art
Today, solar cells provide an energy source to power everything from cars to satellites. Solar cells collect the energy from the sun turning this energy into useful electrical energy. A typical solar cell consists of a very thin disc-shaped wafer of semiconductor material having very thin metal contacts on both the front and back surfaces of this thin wafer. The lateral dimensions of a typical solar cell may range from about 2 cm.times.2 cm to 8 cm.times.8 cm and the thickness of the cell may be about 2 mm. An individual single PN junction solar cell as described above which is made of silicon would typically generate about 0.5 volts (for gallium arsenide solar cells about 0.9 volts). Moreover, no matter how large the solar cell, it will still only generate about 0.5 volts (silicon). Consequently, to achieve a system voltage to power electrical components and motors, which typically require much greater than 0.5 volts, many solar cells must be connected in series until the desired system voltage is achieved. For example, for a satellite having a system voltage of 60 volts, 120 individual solar cells each generating 0.5 volts must be connected together in series.
To connect individual solar cells, contact tabs must be attached to and between all the solar cells needed to complete a solar cell array. These solar cell contact tabs are welded or soldered to the thin contact pads of the respective solar cells, the contact pads typically being about 5 microns thick. In space applications, welding of the contact tabs is preferred over soldering because weld joints can survive long time extreme thermal cycling from the orbital environment, whereas solder joints degrade more readily. The quality of the weld joint must be controlled to achieve a reliable and strong weld which forms a good electrical contact. If the contacts are subjected to an undesirably high temperature, the solar cell semiconductor material could be damaged. On the other hand, if the contacts are subjected to an undesirably low temperature, an incomplete or cold weld may result greatly degrading the performance of the solar cell. In the case where solar cells are employed on satellites or space stations, once the satellite or space station is launched into space any manufacturing defects in any of the solar cells is extremely difficult to fix.
Traditionally welders, such as parallel gap welders, have been used to affix contacts to solar cells. Such parallel gap welders, however, typically have output adjustments for only voltage and pulse duration. Furthermore, there is no means to adjust these parameters on a weld-by-weld basis. From one solar cell to another cell and interconnect contact surface resistance can vary greatly which can effect the total energy actually delivered to the weld joint. Various sensors for indirectly monitoring weld temperature have been used to monitor the welding process. For example, infrared radiation devices collect the infrared rays emitted by the work pieces as they are being heated by the welder in the vicinity of the weld zone.
It would be a great advancement to the art to provide a simple circuit to control the quality of the solar cell welding operation.